Clytus, also known as Cleitarchus, was a Greek historian of the 4th century BCE, chiefly remembered as an early chronicler of Alexander the Great. He was the son of the historian Deinon of Colophon. While some ancient authorities questioned whether he actually accompanied Alexander on his campaigns, tradition holds that he did, and his work was composed in the early Hellenistic period following the king's death. The details of his life are obscure, pieced together from later references; the Byzantine encyclopedia Suda describes him as a writer on Alexander from Alexandria, though this likely reflects his later reputation rather than his place of origin.

His major work was the History of Alexander, a multi-volume account now lost but preserved in fragments and citations by later authors such as Diodorus Siculus, Curtius Rufus, and Plutarch. Clytus holds significant though controversial importance as a source. His history was immensely popular in antiquity, characterized by a vivid and sensationalist style. Despite criticism from contemporaries like Ptolemy and Aristobulus for inaccuracy and exaggeration, his narrative fundamentally shaped the "Vulgate" tradition of Alexander historiography, influencing Diodorus, Curtius, and Justin, and serving as a primary conduit for the Alexander legend.